Air Pollution in Mexico City has been of concern to the city's population and health officials for decades. In the 20th century, Mexico City's population rapidly increased as industrialization brought thousands of migrants from all over the world. Such a rapid and unexpected growth led to the UN declaring Mexico City as the most polluted city in the world in 1992. This was partly due to Mexico City's high altitude (7382 ft above sea level), which causes its oxygen levels to be 25% lower. Carbon-based fuels also do not combust completely. Other factors include the proliferation of vehicles, rapid industrial growth, and the population boom. The Mexican government has several active plans to reduce emission levels which require citizen participation, vehicular restrictions, increase of green areas, and expanded bicycle accessibility. Air pollution causes about one in seventeen (5.9%) of all deaths in the country. It is the eighth largest cause of death, after factors such as diet, overweight, high blood pressure, alcohol and drugs, smoking and lack of exercise. The air pollution of the Mexico City Metropolitan Area, contained within the Valley of Mexico, is measured by the Índice Metropolitano de la Calidad del Aire (Metropolitan Index of Air Quality). History In 1992, the United Nations named Mexico City "the most polluted city on the planet" and "the most dangerous city for children" six years later. From 1950 to 2015, the population in Mexico City increased from three million to twenty million. This population boom occurred mainly because of migrants that were looking for better opportunities, and as a consequence, the industrialization era began. This industrial growth was responsible for emitting over 11,000 tons of waste material into the atmosphere every day. As a result, the economy boomed as did the proliferation of vehicles. In 1980, there were 124 cars and light-duty trucks per 1000 residents. By 2010, there were 267. Population growth, increasing motorization and industrial activities, a constrained basin and intense solar radiation combined to cause intense air-quality problems of both primary and secondary pollutants. The automatic air-quality monitoring network, established in the late 1980s, revealed high concentrations of all criteria pollutants: lead, carbon monoxide, nitrogen dioxide, sulphur dioxide, ozone and particulate matter (PM). Ozone exceeded the air quality standards more than 90 percent of days and peaked above 300 parts per billion (about three times the standard) 40–50 days a year, among the worst in the world.